1. Field of the Invention
The present invention relates to a display panel manufactured by sealing a light emitting element formed on a substrate between the substrate and a covering material. The present invention also relates to a display module in which an IC is mounted on the display panel. Display panels and display modules are herein generically referred to as light emitting devices. The present invention also relates to electric appliances using the light emitting device.
2. Description of the Related Art
Recently, a technique for forming TFTs on a substrate has been remarkably developed, and continues to be developed for its application to active matrix electronic displays. Particularly, TFTs using a polysilicon film can operate at a high speed because such TFTs have higher field effect mobility than TFTs using a conventional amorphous silicon film. Therefore, the control of pixels, which has been conventionally conducted by a driver circuit provided outside a substrate, can be performed by a driver circuit provided on the same substrate on which the pixels are provided.
Such an active matrix electronic display includes various circuits or elements formed on the same substrate. With this structure, the active matrix electronic display provides various advantages such as reduced manufacturing cost, reduced size of a display device including an electronic display as a display medium, an increased yield, and an increased throughput.
Furthermore, among electronic displays, an active matrix light emitting device including a light emitting element as a self-luminescent element has been actively studied. The light emitting device is also called Organic EL Display (OELD) or Organic Light Emitting Diode (OLED).
In contrast with the liquid crystal display device, the light emitting device is self-luminescent. The light emitting element has such a structure that a layer containing an organic compound (hereinafter, referred to as an organic compound layer) is sandwiched between a pair of electrodes (anode and cathode). The organic compound layer generates luminescence by applying an electric field across the pair of electrodes. The organic compound layer has normally a multi-layered structure. As a typical example of the multi-layered structures, a multi-layered structure “hole transport layer/light emitting layer/electron transport layer” proposed by Tang et al. of Kodak Eastman Company is cited. This structure has an extremely high light emitting efficiency. For this advantage, most light emitting devices, which are currently under study and development, employ this structure.
The light emitting element has an anode layer, an organic compound layer and a cathode layer to obtain electro luminescence generated by applying an electric field. The electro luminescence generated from the organic compound layer includes light emission (fluorescence) caused upon transition from a singlet excited state to a ground state and light emission (phosphorescence) caused upon transition from a triplet excited state to a ground state. The light emitting device of the present invention may use any type of light emission.
Furthermore, the light emitting device may have such a multi-layered structure that a hole injection layer, a hole transport layer, a light emitting layer and an electron transport layer are deposited on an anode or a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer are deposited on an anode in this order. Moreover, the light emitting layer may be doped with a florescent pigment or the like.
All layers provided between a cathode and an anode are generically referred to as organic compound layers throughout the specification. Thus, the hole injection layer, the hole transport layer, the light emitting layer, the electron transport layer, the electron injection layer and the like described above are all included in the category of organic compound layers. In this specification, an element constituted by an anode, an organic compound layer and a cathode is referred to as a light emitting element.
As a method of driving a light emitting device, an analog driving method (analog drive) can be given. The analog drive of a light emitting device is described with reference to FIGS. 24 and 25.
FIG. 24 shows a structure of a pixel portion of a light emitting device that is driven in an analog manner. Gate signal lines (G1 through Gy) to which a selecting signal from a gate signal line driver circuit is input are connected to a gate electrode of a TFT 1801 for switching included in each pixel. One of a source region and a drain region of the TFT 1801 for switching included in each pixel is connected to source signal lines (S1 to Sx) to which an analog video signal is input, whereas the other is connected to a gate electrode of a TFT 1804 for current control included in each pixel and a capacitor 1808 included in each pixel.
One of a source region and a drain region of a TFT 1804 for current control included in each pixel is connected to power source supply lines (V1 through Vx), whereas the other is connected to a light emitting element 1806. An electric potential of the power source supply lines (V1 through Vx) is referred to as a power source electric potential. The power source supply lines (V1 through Vx) are connected to the capacitors 1808 included in the respective pixels.
The light emitting element 1806 includes an anode, a cathode and an organic compound layer interposed between the anode and the cathode. Herein, if the anode of the light emitting element 1806 is connected to the source region or the drain region of the TFT 1804 for current control, the anode and the cathode of the light emitting element 1806 are referred to as a pixel electrode and a counter electrode, respectively. On the other hand, if the cathode of the light emitting element 1806 is connected to the source region or the drain region of the TFT 1804 for current control, the anode and the cathode of the light emitting element 1806 are referred to as a counter electrode and a pixel electrode, respectively.
Throughout the specification, an electric potential of the counter electrode is referred to as a counter electric potential. A potential difference between the electric potential at the pixel electrode and the counter electric potential is a light emitting element driving voltage. The light emitting element driving voltage is applied to the organic compound layer.
FIG. 25 shows a timing chart in the case where the light emitting device shown in FIG. 24 is driven in an analog manner. The time period from the selection of one gate signal line until the selection of a next gate signal line is called one line time period (L).
The selection of signal lines (a gate signal line, a first gate signal line, and a second signal line) in this specification means that all the TFTs whose gate electrodes are connected to the signal lines are turned ON.
The time period from the display of one image to another corresponds to one frame time period (F). In the case of the light emitting device shown in FIG. 24, since there are y gate signal lines, y line time periods (L1 to Ly) are provided within one frame time period.
With the enhancement in resolution, the number of line time periods within one frame time period increases. As a result, the driver circuit must be driven at a high frequency.
A power source electric potential at the power source supply lines (V1 through Vx) is held constant, and a counter electric potential at the counter electrodes is also held constant. The counter electric potential has a potential difference with the power source electric potential to such a degree that a light emitting element emits light upon application of the power source electric potential to the pixel electrode of the light emitting element.
During a first line time period (L1), a gate signal line G1 is selected by a selecting signal output from the gate signal line driver circuit to turn ON all the TFTs 1801 for switching connected to the gate signal line G1. An analog video signal is sequentially input to the source signal lines (S1 through Sx). The analog video signal input to the source signal lines is then input to gate electrodes of the TFTs 1804 for current control through the TFTs 1801 for switching.
The amount of a current flowing through a channel formation region of the TFT 1804 for current control is controlled by a gate voltage VGS that is a potential difference between the gate electrode and the source region of the TFT 1804 for current control. Accordingly, the electric potential applied to the pixel electrode of the light emitting element 1806 is determined by the electric potential of the analog video signals input to the gate electrode of the TFT 1804 for current control. Therefore, the light emitting element 1806 is controlled by the electric potential of the analog video signals to emit light.
When the above-described operation is repeated to complete the input of analog video signals to the source signal lines (S1 through Sx), the first line time period (L1) terminates. One line time period may alternatively be constituted by the time period until the completion of input of the analog video signals to the source signal lines (S1 through Sx) and a horizontal blanking time period. Then, a second line time period (L2) starts where a gate signal line G2 is selected by a selecting signal. As in the first line time period (L1), analog video signals are sequentially input to the source signal lines (S1 through Sx) during the second line time period.
When all gate signal lines (G1 through Gy) are selected in this manner, all lines time periods (L1 through Ly) are completed. The completion of all the line time periods (L1 through Ly) corresponds to the completion of one frame period. All pixels perform display during one frame time period to form an image. One frame time period may be alternatively constituted by all line time periods (L1 through Ly) and a vertical blanking time period.
As described above, the amount of light emitted by the light emitting elements 1806 is controlled by the electric potential of analog video signals to perform gray-scale display.
The control of the amount of a current to be supplied to a light emitting element by a voltage between a gate electrode and a source region of a TFT for current control will be described in detail with reference to FIGS. 26A and 26B.
FIG. 26A is a graph showing a transistor characteristic of a TFT. In this graph, a line 401 is referred to as an ID−VGS characteristic (or an ID−VGS curve). Herein, ID indicates a drain current, and VGS indicates an electric potential difference (gate voltage) between a gate electrode and a source region. This graph allows showing the amount of a current that flows at an arbitrary gate voltage.
Normally, for driving a light emitting element, a region defined with a dotted line 402 of the ID−VGS characteristic is used. FIG. 26B shows an enlarged view of the region defined with the dotted line 402.
A shaded region in FIG. 26B is referred to as a saturated region. Actually, the saturated region corresponds to a region from the vicinity of a threshold voltage (VTH) to a gate voltage above the threshold voltage. Within this region, a drain current exponentially changes with respect to a change in gate voltage. In the case of analog driving, a current is controlled by a gate voltage using this region.
A gate voltage of a TFT for current control is determined by an analog video signal which is input to a pixel by turning ON a TFT for switching. At this time, based on the ID−VGS characteristic shown in FIG. 26A, a drain current with respect to a gate voltage is determined in a one-to-one correspondence. More specifically, a voltage of the analog video signal input to the gate electrode of the TFT for current control determines an electric potential of the drain region. As a result, a predetermined amount of a drain current flows into a light emitting element so that the light emitting element emits light in the amount corresponding to the amount of a current.
As described above, the amount of light emitted by the light emitting element is controlled by an analog video signal to perform gray-scale display.
However, the above analog driving has a disadvantage of being extremely affected by variation in characteristics of TFTs. For example, the case where an ID−VGS characteristic of a TFT for switching is different from that of a TFT for switching of an adjacent pixel for displaying the same gray-scale (an ID−VGS characteristic is shifted as a whole to the plus or the minus side) is considered.
In such a case, drain currents of the TFTs for switching differs from each other although such a difference in drain current depends on the degree of variation in characteristics. Therefore, a different gate voltage is applied to a TFT for current control of each pixel. More specifically, a different current flows with respect to each light emitting element. As a result, a different amount of light is emitted from each light emitting element, thereby making it impossible to display the same gray-scale.
Even when the same gate voltage is applied to the TFTs for current control of the respective pixels, the TFTs cannot output the same drain current if there exists a variation in ID−VGS characteristics of the TFTs for current control. Furthermore, as is apparent from FIG. 26A, since the region through which a drain current exponentially changes with respect to a change in gate voltage is used, the amount of a current to be output may greatly vary with a slight shift in ID−VGS characteristic even when the same gate voltage is applied to the TFTs for current control. Under such a condition, even when signals of the same voltage are input, the amounts of light emitted from the light emitting elements of adjacent pixels differ from each other due to a slight variation in ID−VGS characteristic.
Actually, the effect is multiplied by variation in ID−VGS characteristic of the TFTs for switching and that of the TFTs for current control, further complicating the condition for performing the same gray-scale display. As described above, the analog driving is extremely sensitive to a variation in characteristic of TFTs, which is a problem in the gray-scale display of an active matrix light emitting device.